The present invention relates to magnetic ballasts for operating gaseous discharge lamps such as high intensity discharge ("HID") lamps or fluorescent lamps. More particularly, the invention relates to such ballasts which selectively operate the lamps in a reduced light-output or dim mode.
Gaseous discharge lamps such as HID lamps have found widespread acceptance in lighting large outdoor and indoor areas such as athletic stadiums, gymnasiums, warehouses, parking facilities, and the like, because of the relatively high efficiency and low maintenance of HID lamps when compared to other lamp types such as incandescent lamps. When lighting such large areas, there are a number of circumstances in which full illumination of the area is not necessary. For example, in an area which is occupied sporadically, in order to save energy, it is desirable to illuminate the lamps only when the space is sensed to be occupied. The lamps may be extinguished when the space is sensed to be unoccupied, however, because of the slow starting characteristics of HID lamps from an extinguished condition, it is desirable to operate the lamps in a dim mode when full illumination is not desired.
The starting and operating characteristics of HID lamps require the lamps to be operated from a ballast which regulates the lamp operating current and wattage. One common type of ballast used to operate HID lamps is the magnetic ballast comprising a magnetic core having two windings, a first winding connected to an a.c. power source and a second winding connected to the lamp.
In order to operate the lamp in a dim mode from such a magnetic ballast, dimming circuits are used to effect a reduction in power provided by the ballast to the lamp when desired. One type of magnetic ballast commonly used to operate HID lamps is the constant wattage autotransformer. In the constant wattage autotransformer, the two windings of the ballast are capacitively connected. One prior art dimming circuit used with a constant wattage autotransformer operates to switch one or more capacitors either in series with or parallel to the capacitor connecting the windings to alter the impedance of the ballast. Altering the impedance of the ballast effects a change in the power provided to the lamp which thereby effects a change in the brightness of the lamp.
The prior art dimming circuit uses a relay to switch the capacitors into or out of the ballast. One disadvantage of this prior art dimming circuit is that the relay is exposed to high inrush currents, typically about 700 amps, during operation of the circuit. Thus the operating life of the relay is reduced unless a current protection means is added to the dimming circuit. Further, such dimming circuits require the addition of one or more capacitors to the ballast thus raising the manufacturing costs of the ballast. Such prior art dimming circuits are only suitable for use with magnetic ballasts such as the constant wattage autotransformer wherein the windings are capacitively connected.
Another prior art dimming circuit operates with a third winding coupled to the magnetic core. The third winding selectively operates to shunt magnetic flux away from the winding connected to the lamp to thereby reduce the power provided to the lamp. Such prior art dimming circuits are not energy efficient because some power provided to the ballast is shunted away from the lamp. Further, the dimming circuit requires the addition of a winding to the ballast thus adding to the manufacturing costs of the ballasts.
Accordingly, it is an object of the present invention to provide a novel dimming circuit and novel dimming method for a magnetically ballasted HID lamp which obviates the deficiencies of known dimming circuits.
It is another object of the present invention to provide a novel dimming circuit and a novel dimming method for a magnetically ballasted HID lamp which is suitable for use with any type of magnetic ballast.
It is yet another object of the present invention to provide a novel dimming circuit and a novel dimming method for a magnetically ballasted HID lamp which selectively operates the lamp in a dim mode.
It is still another object of the present invention to provide a novel dimming circuit and a novel dimming method for a magnetically ballasted HID lamp which selectively alters the impedance of the ballast to effect a change in the brightness of the lamp.
It is a further object of the present invention to provide a novel dimming circuit and a novel dimming method which may be used with a plurality of magnetically ballasted HID lamps.
It is yet a further object of the present invention to provide a novel dimming circuit and a novel dimming method for a magnetically ballasted HID lamp which selectively operates the lamp in a bright mode responsive to an external stimulus such as the detection of motion or sound.
It is still a further object of the present invention to provide a novel dimming circuit and a novel dimming method for a magnetically ballasted HID lamp which operates the lamp in a bright mode for a predetermined time after power is applied to the ballast, then selectively operates the lamp in a bright mode responsive to the detection of an external stimulus.